Hi All, I know this is a post from a couple of years ago. I'm new to the forum, and I'm answering the question as I have worked for Mclaren building the GT3/GT4 race engines and building/see the development engines. I left Mclaren just over a year ago so now I can write things and help as I am starting my business to specialise with Mclaren engines as I know there issues. And this one is an issue which we have seen a lot and normal on the early cars. The fix only really happen for the 650 as Mclaren used a smaller exhaust valve which allowed Mclaren/Ricardo to have more material between the exhaust valves in the cylinder head which stopped the problem. The problem is that the bridge (the area between the two exhaust valves in the combustion chamber) is too small and overheats due to the amount of material which gets so hot, it cracks between the two valves and then you have the stream/smoke from the exhaust.